Sodium hydroxide solutions are used in petroleum refining and chemical industries to remove hydrogen sulphide from various hydrocarbon streams. When the sulphide has reacted with the sodium hydroxide, the resulting solution is usually referred to as spent sulphidic caustic. Depending on the hydrocarbon source, spent caustics may also contain phenols, mercaptans, amines and other organic compounds that are soluble or emulsified in the caustic. Spent caustics typically have a pH greater than 12 and sulphide concentrations exceeding 2 wt. % (.apprxeq.a more than 0.6 mol/l).
At the moment, spent caustics are usually treated by the "wet air oxidation", wherein sulphides and mercaptans are oxidised chemically at high pressures and temperatures. This process is expensive because of the required chemicals, and leads to residual waste in the form of gaseous sulphur dioxide and liquid sulphuric acid and sulphate. Another known method of disposal of spent caustic is deep well injection, which is also expensive.
A biological process for the treatment of spent caustics was described by Rajganesh, Sublette, Camp and Richardson, Biotechnol. Prog. 1995 (11), 228-230. In this process, sulphides are completely oxidised to sulphate by Thiobacillus denitrificans. However, the production of only sulphate is often not desirable because the pH may become too low. This known process also requires nitrate, which has to be added to the spent caustics, leading to additional costs for chemical requirements.